Covalently cross-linked polymers, commonly referred to as thermosets, are widely used in applications ranging from automobile, truck and tractor tires to high technology aerospace materials. Because such conventional cross-linked polymers have strong covalent bonds which result in fixed, covalent networks of chains, it is not possible to change the shape or molecular structure of such materials after curing (or “setting”). Thus, unlike thermoplastic polymers, which can be reprocessed in the melt state, conventional thermoset polymers cannot be remelted and recycled for high-value use. For example, recycling of cross-linked rubber tires has been a major unmet need in the art.
Over the last decade, attempts have been made to incorporate reversible covalent bonds into polymers whose primary structures, properties, and even shapes can then be changed through molecular reversible rearrangement reactions in response to an external or environmental stimulus. A variety of different chemistries and resulting reversible covalent bonds have been considered, including reversible cycloaddition reactions (e.g., Diels-Alder reactions), exchange reactions (e.g., transesterification reactions, disulfide exchange reactions, siloxane exchange reactions) and other stable free radical mediated reshuffling reactions (e.g., addition-fragmentation reactions of trithiocarbonates and allyl sulfide). All have met with limited practical utility. More specifically, previous studies on reprocessable cross-linked polymers have shown such systems typically require complicated monomer synthesis and/or addition of small molecular components to introduce reprocessability. Furthermore, all such prior art reactions require multiple synthetic steps.
Alternatively, nitroxide-mediated polymerization (NMP) has been investigated. For instance, using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), resulting alkoxyamine-based dynamic covalent bonds can maintain an equilibrium between radicals and dormant species through bond dissociation and recombination under proper conditions. In the context of hydrogel synthesis, it was demonstrated in the literature that alkoxyamine units possess the reversibility in polymer networks by decross-linking the cross-linked polymers with these dynamic covalent bonds incorporated using small molecules that also contain alkoxyamine bonds. However, polymer network synthesis was based on low temperature polymerization to protect the alkoxyamine bonds during the reaction, and gelation was induced by either pre-synthesized divinyl cross-linkers or post-polymerization treatments. In another reported study, polymer synthesis was unduly complicated and the resulting materials were lacking in application value. Accordingly, the reprocessing of cross-linked thermoset polymers remains an ongoing concern in the art. The search continues for a practical, effective approach to the synthesis and recycling of such materials.